1. Field of Invention
The current invention relates to radiation therapy systems, and more particularly external beam radiation therapy systems.
2. Discussion of Related Art
Radiation therapy is the medical use of radiation to treat malignant cells, such as cancer cells. This radiation can have an electromagnetic form, such as a high-energy photon, or a particulate form, such as an electron, proton, neutron, or alpha particle.
By far, the most common form of radiation used in practice today is high-energy photons. Photon absorption in human tissue is determined by the energy of the radiation, as well as the atomic structure of the tissue in question. The basic unit of energy used in radiation oncology is the electron volt (eV); 103 eV=1 keV, 106 eV=1 MeV. Three interactions can be involved in photon absorption in tissue: the photoelectric effect, Compton effect, and pair production.
In the photoelectric effect, an incoming photon transfers energy to a tightly bound electron. The photon transfers practically all of its energy to the electron and ceases to exist. The electron departs with most of the energy from the photon and begins to ionize surrounding molecules. This interaction depends on the energy of the incoming photon, as well as the atomic number of the tissue; the lower the energy and the higher the atomic number, the more likely that a photoelectric effect will take place. The energy range in which the photoelectric effect predominates in tissue is about 10-25 keV.
The Compton effect is the most important photon-tissue interaction for the treatment of cancer. In this case, a photon collides with a “free electron,” i.e., one that is not tightly bound to the atom. Unlike the photoelectric effect, in the Compton interaction both the photon and electron are scattered. The photon can then continue to undergo additional interactions, albeit with a lower energy. The electron begins to ionize with the energy given to it by the photon. The probability of a Compton interaction is inversely proportional to the energy of the incoming photon and is independent of the atomic number of the material. The Compton effect dominates in the range of ˜25 keV-25 MeV and is therefore the most common interaction occurring clinically, as most radiation treatments are performed at energies of about 6-20 MeV.
In pair production, a photon interacts with the nucleus of an atom. The photon gives up energy to the nucleus and, in the process, creates a positron-electron pair of particles. The positive electron (positron) ionizes until it combines with a free electron in positron-electron annihilation. This positron-electron annihilation generates two photons that travel in opposite directions. The probability of pair production is proportional to the logarithm of the energy of the incoming photon and is dependent on the atomic number of the material. The energy range in which pair production dominates is ≧25 MeV. This interaction occurs to some extent in routine radiation treatment with high-energy photon beams.
With the advent of high-energy linear accelerators, electrons have become a viable option in treating superficial tumors up to a depth of about 5 cm. Electron depth dose characteristics are unique in that they produce a high skin dose but exhibit a falloff after only a few centimeters.
Electron absorption in human tissue is greatly influenced by the presence of air cavities and bone. The most common clinical uses of electron beams include the treatment of skin lesions, such as basal cell carcinomas, and boosting of areas that have previously received photon irradiation, such as postoperative lumpectomy or mastectomy scar in breast cancer patients, as well as select nodal areas in the head and neck.
Fast, accurate dose computation algorithms are important for radiation therapy planning as they are the only available method of ensuring that the desired dose is delivered to a specific patient. Dose computation includes two parts: a source model and a transport model. The source model provides the incident fluence. The transport model computes the dose that results from the incident fluence and is currently the performance bottleneck. The three main transport algorithms in the order of increasing accuracy/decreasing performance are pencil beam, superposition/convolution, and Monte Carlo. Superposition/convolution is the current clinical standard method of calculating radiation dose for external beam radiation therapy.
In recent years, treatment quality has been increased by the use of intensity modulation. This technique uses a multi-leaf collimator to define multiple apertures from a single beam direction providing the ability to vary the intensity of radiation across the beam. This technique allows us to conform radiation treatment to the shape of the target and avoid critical structures while drastically increasing the number of beam parameters. In order to determine the best set of multi-leaf collimator settings, the treatment planning system must optimize, through multiple iterations of dose calculations, an objective function having the drastically increased number of beam parameters. In practice, the treatment planner repeats the optimizations multiple times in order to achieve the best results possible for the patient. Therefore, while a single optimization may take five minutes for a set of five beams, the entire process may take several hours to produce a clinically acceptable plan. This limits both the quantity and quality of intensity modulated plans in clinical work flow.
This clinical workflow limitation extends to more complex techniques such as volumetric modulated arc therapy (Otto, K., Med. Phys. 35, 310-317, 2008), intensity modulated arc therapy (Yu, C. X., Phys. Med. Biol. 40, 1435-1449, 1995), and adaptive radiation therapy (Yan, D., Vicini, F., Wong, J., Martinez, A, Phys. Med. Biol. 42, 123-132, 1997). Furthermore, this clinical workflow limitation prohibits real-time radiation therapy; the ability to scan, re-plan and treat every patient daily. A thorough review of dose calculation in radiation therapy is available from Ahnesjo et al. (Ahnesjo, A., Aspradakis, M, Phys. Med. Biol. 44, R99-R155 1999).
Thus, computational performance of the dose computation is a limiting factor in radiation therapy treatment plan quality. Traditionally, improvements in treatment quality have been realized by faster hardware. But, Moore's law has changed. Instead of doubling in speed every 18 months, computers are doubling the number of processing cores. And as processors are becoming multi-core, the many-core architectures of graphics processing units (GPUs) are gaining the flexibility to run general purpose algorithms. To realize the promised performance gains from the recent trend in computer hardware, traditional serial algorithms used in radiation dose computation should be replaced with parallel ones. Recently, Nucletron corporation made an announcement regarding GPU acceleration in their treatment planning system, though published details are not yet available. However, straight-forward partitioning of the existing serial algorithms to produce multiple threads for multiple processing cores does not work. This is because the threads are farmed out to calculate the same radiation dose based on the same input data and read/write conflicts may easily arise. When a write-on-write (WOW) conflict arises, for example, it can happen that only the last write is stored, leading to inaccurate dose calculations. Thus, there is a need in the art for improved dose calculation by utilizing parallel computing on multiple processing cores. There also remains a need for real-time dose calculation.